1. Field of the Invention
The present invention relates to control of cell discarding that occurs due to congestion, etc., between communication nodes in asynchronous transfer mode (ATM) communication, and more particularly to a cell-discard control apparatus that makes it possible to discard voice cells without degrading voice articulation during voice communication. Furthermore, the invention relates to a cell-discard control apparatus that is also capable of minimizing degradation of image information due to cell discarding.
2. Description of the Prior Art
In recent years, B-ISDN (Broadband Integrated Service Digital Network) has been emerging as the next generation public network, and with the deployment of the B-ISDN, even more flexible broadband communication networks than before are being realized which provide such differing services as voice communications, very high speed file transfer, information communications, communications between LANs, moving images, and even moving image services of high-definition television (HDTV). ATM communication technology that can handle such multimedia is used in the B-ISDN.
A user terminal is connected to a communication node via a user network interface (UNI). Each communication node is connected to other communication nodes via node network interfaces (NNIs) and digital service units (DSUs) for high speed asynchronous transmission of ATM cells. Each communication node contains ATM adaptation layers (AALs) and ATM switches. The AALs perform ATM cell assembly and disassembly between the user terminals and the ATM switches. Data from each user terminal is broken up into a plurality of ATM cells.
The data from the terminal is broken up into cells by the AAL and a communication path is set up by the ATM switch. Using the header data in each ATM cell, the ATM switch performs high speed switching operation by hardware which is a feature of ATM. If communication path congestion is detected by the ATM switch during communication, the ATM switch discards part of the data transmitted from the transmitting user terminal. In this case, the ATM switch checks the CLP (cell loss priority) control bit in the ATM cell header and preferentially discards cells whose CLP value is "1".
For voice cells which generally have a high level of redundancy and are therefore less affected by cell discarding, the CLP is set to "1", whereas for other cells such as data, modem, facsimile data, etc., the CLP is set to "0".
However, communication path congestion usually occurs in bursts, which, in the prior art example, results in discarding voice cells in bursts as they have the higher cell-discard priority (CLP=1). This causes abrupt occurrence of a temporarily silent condition during voice communication or intermittent interruptions of voice communication, greatly degrading articulation of voice communication.
There has also been the problem during transmission of image information that burst loss of cells causes instantaneous disruption of a television or computer display screen or appears as clearly visible noise on the screen.